Light source for swept source optical coherence tomography based on cascaded distributed feedback lasers with engineered band gaps

ABSTRACT

The present invention is a tunable semiconductor laser for swept source optical coherence tomography, comprising a semiconductor substrate; a waveguide on top of said substrate with multiple sections of different band gap engineered multiple quantum wells (MQWs); a multiple of distributed feedback (DFB) gratings corresponding to each said band gap engineered MWQs, each DFB having a different Bragg grating period; and anti-reflection (AR) coating deposited on at least the laser emission facet of the laser to suppress the resonance of Fabry-Perot cavity modes. Each DFB MQWs section can be activated and tuned to lase across a fraction of the overall bandwidth as is achievable for a single DFB laser and all sections can be sequentially activated and tuned so as to collectively cover a broad bandwidth, or simultaneously activated and tuned to enable a tunable multi-wavelength laser. The laser hence can emit either a single lasing wavelength or a multiple of lasing wavelengths and is very suitable for swept-source OCT applications.

PRIORITY

This application claims the benefit of the filing date under 35 U.S.C.§119 (e) of U.S. Provisional Patent Application Ser. No. 60/714,286,filed Sep. 6, 2005, which is hereby incorporated by reference in itsentirety.

TECHNICAL FIELD OF THE INVENTION

One or more embodiments of the present invention relate generally tolight sources for optical imaging. In particular, the invention is asemiconductor-based tunable laser for swept source optical coherencetomography.

BACKGROUND OF THE INVENTION

Optical Coherence Tomography (OCT) is a technology for performinghigh-resolution cross sectional imaging that can provide images oftissue structure on the micron scale in situ and in real time (U.S. Pat.No. 5,321,501). In recent years, it has been demonstrated that Fourierdomain OCT (FD-OCT), which so far employs either a wavelength sweptsource and a single detector or a broadband source and an arrayspectrometer, has significant advantages in both speed andsignal-to-noise ratio as compared to time domain OCT (TD-OCT) (Choma, M.A. et al. (2003). “Sensitivity advantage of swept source and Fourierdomain optical coherence tomography.” Optics Express 11(18): 2183-2189).In TD-OCT, the optical path length between the sample and reference armsneeds to be mechanically scanned. In both swept source OCT (SS-OCT) andspectrometer-based spectral domain OCT (SD-OCT), the optical path lengthdifference between the sample and reference arm is not mechanicallyscanned. Instead, a full axial scan (also called A-scan) is obtained inparallel for all points along the sample axial line within a short timedetermined by the wavelength sweep rate of the swept source (in SS-OCT)or the line scan rate of the line scan camera (in SD-OCT). As a result,the speed for each axial scan can be substantially increased as comparedto the mechanical scanning speed of TD-OCT and this is especiallybeneficial for real-time imaging of living biological samples such asthe human eye. In addition, SD-OCT and SS-OCT can provide substantiallygreater signal-to-noise ratio relative to TD-OCT, as explained by Mitsui(1999) “Dynamic Range of Optical Reflectometry with SpectralInterferometry.” Japanese Journal of Applied Physics 38(10): 6133-6137.

SS-OCT can be achieved using either a single lasing wavelength tunablelaser or a multiple lasing wavelengths tunable laser. FIG. 1 shows thebasic configuration of a SS-OCT system based on a tunable laser with asingle lasing wavelength. Light from a tunable single-wavelength laser102 is split through a beam splitter or fiber coupler 104 into areference arm 106 and a sample arm 108 of an interferometer and theinterference signal is detected with a single high-speed photodetector110. By sweeping the wavelength of the monochromatic source 102, theinterference spectrum from the OCT interferometer is recordedsequentially. The axial reflectance distribution of the sample isobtained by a Fourier transform of the sequentially acquired spectralinterference signal.

FIG. 2 shows a system, described in a co-pending US patent applicationby Zhou and Everett (“Fourier domain optical coherence tomographyemploying a swept multi-wavelength laser and a multi-channel receiver”filed on Jul. 1, 2005, application Ser. No. 11/174,158) incorporatedherein by reference, of a SS-OCT system based on a tunable laser withmultiple lasing wavelengths. Light from a tunable multi-wavelength laser202 is split via a beam splitter, for example, fiber coupler 204, into areference arm 206 and a sample arm 208 of an interferometer. Lightreturning from the reference arm and the sample is combined, either withthe same splitter as shown in FIG. 2 or another beam combining elementas is known in the art of interferometry. The combined, interfered lightis sent to a detector, in this case, multi-channel receiver 210. Aprocessor 220 obtains the spectral interferogram data from themulti-channel receiver 210, synchronized with the sweeping of themulti-wavelength laser 202. It combines the data samples from theindividual channels to form the full spectral interference fringes andcarries out a Fourier transform of the spectral interference fringes toprovide the information of the reflectance distribution along the depthwithin the sample 222.

A practical SS-OCT system requires a high speed swept source with asweep rate of at least about 20 kHz that is continuously tunable over abroad tuning range (preferably greater than 50 nm). Current commerciallyavailable tunable lasers can be divided into electronically tuned lasersand mechanically tuned lasers. Electronically-tuned lasers are eitherlimited in their tuning range (typically 5 nm to 10 nm for a singledistributed feedback (DFB) laser), or discretely tunable in order tocover a wider range as in the case of sampled grating distributedfeedback reflector (SG-DBR) lasers (see for example, U.S. Pat. No.4,896,325, U.S. Pat. No. 5,325,392). The discretely tunable lasersdescribed in U.S. Pat. No. 4,896,325 and U.S. Pat. No. 5,325,392 operateusing a single gain section, and they tune using a Vernier effectbetween the two DBR end mirrors, so both DBR end mirrors and aphase-matching device must be simultaneously continuously tuned toproduce discrete tuning; these features make this design inconvenientfor SS-OCT. Most mechanically tunable lasers are slow. Some use fiberand piezo based Fabry-Perot (FP) filters (see for example Huber, R. etal. (2005) Optics Express 13(9): 3513-3528; and (2006) Optics Express14(8): 3225-3237) and others use fast rotating polygon mirrors (see forexample, US20050035295). For example, patent application US20050035295and the article by Oh, W. Y. et al. (“Wide tuning range wavelength-sweptlaser with two semiconductor optical amplifiers.” Photonics TechnologyLetters, IEEE 17(3): 678-680) disclosed a wavelength tuning source forSS-OCT that employs a continuously rotating optical arrangement forlasing wavelength selection. The current price of a swept sourcesuitable for OCT is very high (see for example, Thorlab Inc. ProductCatalog, Vol. 17, (2005) page 469) and in addition, the demonstratedwavelength sweep rate is limited to about 20 kHz.

On the other hand, tunable semiconductor lasers developed for opticalfiber communications either are step-tuned to fit the ITU grid (see forexample, Amano, T. et al. (2005). “Optical frequency-domainreflectometry with a rapid wavelength-scanning superstructure-gratingdistributed Bragg reflector laser.” Applied Optics 44(5): 808-816) or,if continuously tunable, are very slow (see for example, U.S. Pat. No.6,847,661) and they do not meet the requirement for an SS-OCT system,such as the high wavelength sweeping rate (more than 20 kHz) and thebroad spectral range to be covered (e.g. 25 to 200 nm). Although thereare various designs of semiconductor based tunable lasers (see forexample, Muller, M. et al. (2003) “1.3-μm Continuously TunableDistributed Feedback Laser With Constant Power Output Based onGalnNAs—GaAs”, Photonics Technology Letters, IEEE 15(7) 897-899; Buss J.et al. (2005) “Tunable Laser Diodes and Related Optical Sources” SecondEdition, John Wiley & Sons, Inc., Hoboken, N.J., and others as cited inthis application, which are all incorporated in their entirety herein byreference), these lasers are not designed specifically for SS-OCTapplications. In particular, there are attempts to cascade a fewdistributed feedback (DFB) semiconductor lasers along a single channelwaveguide to achieve complex coupled DFB lasers (see for example, U.S.Pat. No. 5,936,994; U.S. Pat. No. 6,104,739; U.S. Pat. No. 6,201,824;Hong, J. et al. (1998) “Enhanced Wavelength Tuning Range in Two-SectionComplex-Coupled DFB Lasers by Alternating Gain and Loss Coupling”,Journal of Lightwave Technology, 16(7): 1323). When the individualsections of these lasers are built on semiconductor structures havinguniform energy band gap, there is a significant overlap of the opticalgain curve associated with each DFB grating and the resulting lasershave limited tuning range.

In light of the above, there is hence a need in the art for a low costcontinuously tunable laser that meets the requirement of a real timeSS-OCT system.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows the basic configuration of a SS-OCT system based on atunable laser with a single lasing wavelength;

FIG. 2 shows the basic configuration of a SS-OCT system based on atunable laser with multiple lasing wavelengths;

FIG. 3 shows a first embodiment of the design of the invented cascadedmulti-DFB based semiconductor band gap engineered laser for swept sourceOCT;

FIG. 4 shows a second embodiment of the design of the invented cascadedmulti-DFB based semiconductor band gap engineered laser for swept sourceOCT;

FIG. 5 shows the flexibility of the relationship between the tunablelasing wavelength(s) and the tuning time;

FIG. 6 shows a scheme to monitor the lasing wavelength(s) so that thelasing wavelength versus time relationship can be calibrated in realtime.

DETAILED DESCRIPTION

The present invention is a new design for a widely continuously tunablesemiconductor laser that can emit either a single lasing wavelength ormultiple lasing wavelengths, which is very suitable for swept-source OCTapplications. As shown in FIG. 3, in accordance with one or moreembodiments of the present invention, the new design consists of amultiple number of tunable DFB lasers cascaded along a single waveguidewith each section of the multiple quantum wells (MQWs) (corresponding toeach DFB grating) having a different energy bandgap so that along thelaser emission direction, the follow-up sections are significantlytransparent to light emitted from the previous sections. Each DFB MQWssection can be activated and tuned to lase across a fraction of theoverall bandwidth as is achievable for a single DFB laser and allsections can be sequentially activated and tuned so as to collectivelycover a broad bandwidth. MQWs are the preferred gain medium for use inthis laser, but any optical gain medium can be used if the wavelengthrange of gain can be adjusted between individual gain sections. Singlequantum wells, semiconductor heterojunctions, or quantum dots arealternative practical gain media. Alternatively, all sections can alsobe simultaneously activated and tuned to enable a tunablemulti-wavelength laser.

The presently invented laser structure can be realized using twowell-established technologies. The gain medium with sections ofdifferent band gap can be produced by semiconductor band gap engineeringthrough quantum well intermixing (see for example U.S. Pat. No.6,617,188), or selective area growth or regrowth (see for example T. vanCaenegem et al, Progress in Crystal Growth and Characterization ofMaterials 35(2-4): 263-268). The relatively narrowly-tunable laserresonators are produced using existing DFB laser technology. Bycascading a number of tunable DBF lasers along a single waveguide, thecost of such a device can be substantially reduced as compared to thatof the mechanically tunable lasers. Although, each DFB laser section canonly be tuned over a relatively narrow range (e.g. 5 nm), bysequentially activating and tuning each section, the combined tuningrange can hence be greatly increased. (For example, 10 sections canprovide a 50 nm tuning range). In addition, the achievable tuning speedcan be many orders of magnitude higher than that of the mechanicalcounterpart.

In a first preferred embodiment of FIG. 3, the laser is made on asemiconductor substrate 302 that has a waveguide 304 with multiplesections of different band-gap-engineered multiple quantum wells (MQWs)306. The multiple distributed feedback gratings 308 corresponding to theband-gap-engineered MWQs 306 are fabricated either on top of or belowthe waveguide 304 as shown in FIG. 3, each section has a different Bragggrating period. The quantum well corresponding to each section can haveits energy band gap shifted, preferably by a band-gap engineeringtechnique that can be performed after deposition of epitaxial layers ofsemiconductor; quantum well intermixing is one such post-epi technique.Suppose we use the lasing output on the right hand as shown in the FIG.3, we can have the peak luminescence wavelengths associated with thedifferent MQW energy band gaps to be λ₁>λ₂>λ₃> . . . >λ_(n) (from leftto right) to avoid re-absorption of emitted laser light from previoussections on the left of each section. Here, the band gap energy of eachsection increases from left to right. There can be a common bottomcathode electrode 312 electrically connected to the substrate 302 andseparate anode electrodes 314 for each section. Each section can beactivated and tuned by injecting current which will both pump the gainmedium and also tune the effective grating period and hence the lasingwavelength. One can control the rate of current injection in order tovary the carrier densities in the substrate near the Bragg grating,which in turn varies the effective refractive index within the Bragggrating, which in turn creates a variable effective grating period.Alternatively, each section can also be electrically pumped with aconstant current and thermally tuned, by respectively injecting aconstant electrical current which will pump the gain medium and varyingthe temperature which will tune the effective grating period and hencethe lasing wavelength, although thermal tuning will be much slower ascompared to electrical tuning. If there is some degree of overlap of thegain curve between or among some neighboring sections, those follow-upsections in the right hand side portion, closer to the laser emissionfacet, can be pumped to transparency or to have optical gain but belowlasing threshold to ensure that the emitted laser light is not absorbed.Anti-reflection (AR) coating 310 is preferably deposited on at least thelaser emission facet or on both facets of the laser to suppress theresonance of Fabry-Perot cavity modes. In this embodiment of FIG. 3, thelasing power may change with the lasing wavelength, but this isacceptable for SS-OCT application because the instantaneous power can bemeasured and the interference spectrum can be compensated during dataprocessing for the power variation.

In a second preferred embodiment as shown in FIG. 4, the distributedfeedback structures 408 are made on the two sides of the waveguide 404.Separate electrodes can be used, with the pump electrode 414 on thewaveguide 404 for electrically pumping the gain medium, and the otherwavelength tuning electrode 416 injecting carriers on the two sides ofthe waveguide 404 to modulate the refractive index and control theeffective grating period. The advantage of this embodiment is that theinjection current for laser power control can be substantially separatedfrom the electrical current for lasing wavelength tuning, so that thelasing power can be held substantially constant as the lasing wavelengthis tuned. Nearly-constant power gives nearly-uniform signal-to-noiseratio in the interference spectrum, which helps to achieve both goodaxial resolution and good signal-to-noise ratio in reconstruction of thereflectance distribution. Like the FIG. 3 embodiment, the FIG. 4embodiment can include a bottom cathode electrode 412 on the substrate402. In addition, one or both of the ends of the waveguide 406 caninclude an AR coating 410.

One can have all the sections lase and hence sweep the multiplewavelengths together as a multi-wavelength laser source. In this case,the electrical pumping anode electrodes (314 for embodiment 1 and 414for embodiment 2) can be made into one anode electrode to reduce thenumber of electrical connection pins for the laser. On the other hand,one can turn the sections on one at a time, sweeping each section forsimple detection. In the latter case, for the second preferredembodiment, the wavelength tuning electrode 416 can be combined to onefor all the gratings to reduce the electrode number, while each pumpingelectrode needs to be activated sequentially.

Suppose we have 20 sections, with each section 500 μm long, the outputlasing power from each section can reach 10 to 40 mW and the completelaser die is 10 mm long which is feasible.

Note that there can be dead time 502 between turning one section off andturning the next section on as is shown in FIG. 5, but this isacceptable as long as we know the lasing wavelength and power at aspecific time, because we can use data processing to drop the signalduring the dead time 502. The tuning range 504 of each section can alsooverlap with that of the next section, as we can use data processing todrop or average the measurements in the duplicated or overlappedwavelength range 506. The tuning curve 508 also does not need to belinear, as long as we know the shape, we can compensate it in dataprocessing.

Often one uses an auxiliary interferometer to monitor the wavelengthsweep (for example, FIG. 5 of U.S. Pat. No. 5,956,355). Often theauxiliary interferometer is a Fabry-Perot etalon that provides a seriesof peaks in transmitted power, these peaks being uniformly spaced ininverse wavelength. In this multiple-section laser, the firsttransmission peak traversed by each section must be identified fromamong the many transmission peaks of the auxiliary interferometer. Therelationship of wavelength λ versus time t of each section may be stableenough such that the identity of the first peak of a given section canbe determined once by a wavelength measurement during initialcalibration. If the sweep relationship is not that stable, one can use asecond auxiliary interferometer with a different spacing of transmissionpeaks to uniquely identify the starting wavelength of each section basedon its relationship to two incommensurate sets of transmission peaks.

FIG. 6 shows such a scheme to monitor the light source so that thelasing wavelength versus time relationship can be calibrated in realtime. The tunable light source 602 is powered by electrical power source601 and linked to the OCT system 610 through, for example, a fiber. Asmall fraction of light is tapped by fiber coupler 604 to Fabry-Perotfilters 606 and 608, these filters having etalon optical gaps of d₁ andd₂ respectively. The optical power transmitted through filters 606 and608 can be measured by photodetectors 612 and 614 respectively. EachFabry-Perot etalon will provide a series of peaks in transmitted power,but the spacing of these peaks differs for filters 606 and 608 due totheir different gap spacing. The combined pattern of transmission fromboth filters can be used to uniquely identify the wavelength of a giventransmission peak, and thus to determine the wavelength of the sweptsource 602.

The presently invented tunable semiconductor laser source is especiallyuseful for SS-OCT applications. Meanwhile, the presently invented lightsource is also useful for other applications including sensing,spectroscopy and metrology.

Although various embodiments that incorporate the teachings of thepresent invention have been shown and described in detail herein, thoseskilled in the art can readily devise many other varied embodiments thatstill incorporate these teachings.

The following references are hereby incorporated by reference.

-   US Patent Documents-   U.S. Pat. No. 4,896,325-   U.S. Pat. No. 5,321,501-   U.S. Pat. No. 5,325,392-   U.S. Pat. No. 5,936,994-   U.S. Pat. No. 5,956,355-   U.S. Pat. No. 6,104,739-   U.S. Pat. No. 6,201,824-   U.S. Pat. No. 6,617,188-   U.S. Pat. No. 6,847,661 US20050035295-   US patent application, Yan Zhou and Matthew J. Everett “Fourier    domain optical coherence tomography employing a swept    multi-wavelength laser and a multi-channel receiver” filed on Jul.    1, 2005, application Ser. No. 11/174,158

OTHER PUBLICATIONS

-   Amano, T. et al. (2005). “Optical frequency-domain reflectometry    with a rapid wavelength-scanning superstructure-grating distributed    Bragg reflector laser.” Applied Optics 44(5): 808-816-   Buss J. et al. (2005) “Tunable Laser Diodes and Related Optical    Sources” Second Edition, John Wiley & Sons, Inc., Hoboken, N.J.-   Choma, M. A. et al. (2003). “Sensitivity advantage of swept source    and Fourier domain optical coherence tomography.” Optics Express    11(18): 2183-2189-   Hong, J. et al. (1998) “Enhanced Wavelength Tuning Range in    Two-Section Complex-Coupled DFB Lasers by Alternating Gain and Loss    Coupling”, Journal of Lightwave Technology, 16(7): 1323-   Hong, J. et al. (1999) “Cascaded strongly gain-coupled (SGC) DFB    lasers with 15-nm continuous-wavelength tuning”, Photonics    Technology Letters, IEEE 11(10): 1214-1216.-   Hong, J., et al. (1999) “Matrix-grating strongly gain-coupled    (MC-SGC) DFB lasers with 34-nm continuous wavelength tuning range”    Photonics Technology Letters, IEEE 11(5): 515-517.-   Huber, R. et al. (2005). “Amplified, frequency swept lasers for    frequency domain reflectometry and OCT imaging: design and scaling    principles.” Optics Express 13(9): 3513-3528-   Huber, R., et al. (2006). “Fourier Domain Mode Locking (FDML): A new    laser operating regime and applications for optical coherence    tomography.” Optics Express 14(8): 3225-3237-   Muller, M. et al. (2003) “1.3-μm Continuously Tunable Distributed    Feedback Laser With Constant Power Output Based on GalnNAs—GaAs”,    Photonics Technology Letters, IEEE 15(7) 897-899;-   Mitsui T. (1999) “Dynamic Range of Optical Reflectometry with    Spectral Interferometry.” Japanese Journal of Applied Physics    38(10): 6133-6137-   Oh, W. Y. et al. (“Wide tuning range wavelength-swept laser with two    semiconductor optical amplifiers.” Photonics Technology Letters,    IEEE 17(3): 678-680-   Thorlab Inc. Product Catalog, Vol. 17, (2005) page 469-   T. van Caenegem et al, Progress in Crystal Growth and    Characterization of Materials 35(2-4): 263-268 (1997)

1. A swept source OCT system comprising: a tunable light source; a beamsplitter for dividing the light along a sample and a reference path; aphotodetector for receiving light returned from both the sample and thereference paths and generating output signals as a function of time asthe wavelength of the source is tuned; a processor for analyzing theoutput signals to derive a reflectance distribution along the samplepath and wherein the tunable light source includes an elongated opticalwaveguide structure, with one end thereof defining a laser output facet;a linear series of distributed feedback gratings formed along thewaveguide structure to define a series of resonant cavities; a series ofsemiconductor gain structures formed within the waveguide structure andaligned with the gratings, with the bandgap energy of the gain structureincreasing towards said output facet; and a power supply for supplyingcurrent to the gratings and the gain structures in a manner to generatelaser output from each resonant cavity and for wavelength tuning theoutput.
 2. An OCT system as recited in claim 1, wherein a common currentis supplied to a grating and the associated gain structure.
 3. An OCTsystem as recited in claim 1, wherein current is independently suppliedto a grating and the associated gain structure.
 4. An OCT system asrecited in claim 1, wherein current is simultaneously supplied to allthe gratings and the gain structures and wherein the current is tuned sothat a tuned, multi-wavelength output is generated.
 5. An OCT system asrecited in claim 1, wherein current is supplied to the gain structure ofeach of the associated resonant cavities, said current being controlledin manner so that each of the gain sections are sequentially activatedand tuned one at a time so that a tuned, narrow-band output isgenerated.
 6. A method of evaluating the reflectance distribution withina sample using swept source OCT comprising the steps of: providing atunable light source, said light source including an elongated opticalwaveguide structure, with one end thereof defining a laser output facet,a linear series of distributed feedback gratings formed along thewaveguide structure to define a series of resonant cavities, a series ofsemiconductor gain structures formed within the waveguide structure andaligned with the gratings, with the bandgap energy of the gainstructures increasing towards said output facet, and a power supply forsupplying current to the gratings and the gain structures in a manner togenerate laser output from each resonant cavity and for wavelengthtuning the output; dividing the light along a sample and a referencepath; measuring light returned from both the sample and the referencepaths and generating output signals as a function of time as thewavelength of the source is tuned; and analyzing the output signals toderive a reflectance distribution along the sample path and wherein thelight source is operated by supplying current to the gratings and thegain structures in a manner to generate laser output from each resonantcavity and for wavelength tuning the output.
 7. A method as recited inclaim 6, wherein current is simultaneously supplied to all the gratingsand the gain structures and wherein the current is tuned so that atuned, multi-wavelength output is generated.
 8. A method as recited inclaim 6, wherein current is supplied to the gain structure of each ofthe associated resonant cavities, said current being controlled inmanner so that each of the gain sections are sequentially activated andtuned one at a time so that a tuned, narrow-band output is generated.